Water discipline is the practice of controlling one's water intake. There are two main kinds of water discipline; making sure one gets plenty of water, or strategically depriving oneself of water.
It is generally accepted that one should drink eight glasses of water a day (though that may be wrong; see previous hardlink for a discussion of this). The United States military advocates a more complicated system (of course) which involves terrain, temperature, and how much you're working. It seems like a sensible system, at least. A basic chart of how much water one should drink can be found here.
This form of water discipline also involves boiling or otherwise purifying all water that will enter your system, and using a very bare minimum for washing. These two precautions should be taken by most people, however, and barely bear mentioning as "water discipline" measures.
The other view of what water discipline means is embodied by the Fremen in Frank Herbert's Dune. The Fremen use stillsuits to create a self-contained environment in which water loss is kept to a minimum. The Fremen also contained very little body water, but maintained full mobility and strength (and made fun of offworlders for "water fat", to boot). The Fremen, however, are fictional.
For real people, intentional dehydration is chancy, but useful. Suddenly drinking less water will lead to water retention and bloatedness, but weaning oneself off the eight cups per day plan over the course of a few months is possible. In a less active lifestyle, where the maximum of exertion is walking seven blocks with a twenty pound backpack, drinking two to three cups a day can be sufficient. When one becomes sick or for other reasons wishes to purge toxins from one's body, drinking eight cups a day causes a massive cleansing of the system.
This is not medical advice. It is e2. Talk with your doctor before starving your body of a necessary substance.